Devices for detecting stack height of sheets are known in the art. In EP-0 768 263-A1, a conventional device for detecting stack height is disclosed. According to this prior art development, means is provided for detecting a stack height in at least one collecting tray of an output unit of a printer. More particularly, a feeler is provided in the form of a pivotable feeler bracket movable cyclically against the sheet stack into a sensing position. At some position above the sheet stack and in the stacking direction, a sensor in the form of an opto-electronic sensor senses the position of the feeler and detects the sheet stack height. The feeler bracket in the aforementioned prior art development has at a first end a feeler/pressure finger pivoted by means of a microprocessor-controlled reciprocating magnet about its center rotation axis. The reciprocating magnet is joined via a spring element at its reciprocating armature to a second end of the feeler bracket located opposite the first, at which a switching tab for actuating the sensor is also arranged. The feeler bracket, sensor, and reciprocating magnet are arranged outside the collecting tray and in front of the end-surface wall of the collecting tray, i.e. the front alignment edge for the paper sheets delivered into the collecting tray. With the feeler bracket in an initial position, the feeler finger is located above and out of engagement with the front region of the sheet stack, and the switching tab is outside the sensing region of the sensor. During the sensing cycle, the feeler bracket pivots through a slot in the end-surface wall into the collecting tray and, with the feeler finger, onto the sheet stack, whereby the switching tab pivots in the direction of the sensor. Not until the maximum permissible stack height is reached is the sensor actuated or covered by means of the switching tab, and a signal is sent to the control unit to stop sheet in-feed. Alternative embodiments have sensor means (multiple sensors) or switching tabs (with multiple slots) for detecting intermediate values of the sheet stack height. To detect a first sheet in the collecting tray, a further sensor is arranged in its bottom or deposition surface.
A major shortcoming of the above and other similar existing sheet stack height detecting devices is that the reciprocating magnet generates severe vibration and noise. Further, such devices require complex sensor means to detect exact intermediate values of the sheet stack height and to detect the first sheet in the collecting tray. A further shortcoming of existing developments is that the means for detecting the stack height are located in the region of a possible transport path for the completed sheet stack, or in the removal/input region of the collecting tray.
Therefore, a need persists in the art for a device for detecting stack height of sheets stacked in an input and/or output tray of an apparatus that is compact, and simple to construct, cost effective to manufacture and relatively simple to operate.